1. Field of the Invention
The present invention relates to an optical system such as one suitable as a projection optical system for use in an image projection apparatus (projector), which enlarges and projects on a screen an image formed by an image display element.
2. Description of the Related Art
An image projection apparatus (projector) enabling an image in a personal computer or a video to be projected on a large screen has been widely used for presentations and movie viewing. A projection optical system for use in the projector is required to have a wide angle of view, have little distortion, and provide a high-quality projection image with no color bleeding (no lateral chromatic aberration (chromatic aberration of magnification)). The projection optical system for use in the projector is also required to have a long back focus and be telecentric on a projected side.
As an optical system advantageous to satisfaction of these requirements, a retro-focus-type optical system has been known. The retro-focus-type optical system includes a front lens group having negative refractive power, an aperture stop, and a rear lens group having positive refractive power. In this optical system, since refractive power arrangement with respect to the aperture stop is asymmetric, various kinds of aberration, especially, lateral chromatic aberration, tend to be generated significantly. As the retro-focus-type optical system, there has been known an optical system for an image projection apparatus in which a material with significant chromatic dispersion is used for a negative lens of the front lens group to correct lateral chromatic aberration (Japanese Patent Application Laid-Open No. 2010-139766).
The retro-focus-type optical system has been widely used as a projection optical system for an image projection apparatus (projector) since the retro-focus-type optical system easily achieves a wide angle of view and relatively easily obtains a long back focus. However, in the retro-focus-type optical system, since refractive power arrangement with respect to the aperture stop is asymmetric, distortion and lateral chromatic aberration are generated significantly. Thus, favorable correction of the various kinds of aberration is a major issue.
An effective way to correct distortion in the retro-focus-type optical system is to arrange a positive lens in the front lens group at a position in which the height of incidence of a peripheral ray is high to generate positive distortion by a ray raising effect. At this time, lateral chromatic aberration is also corrected. However, since lateral chromatic aberration has a different correction effect depending on a wavelength thereof, the light having a short wavelength tends to be overcorrected.
To solve this issue, in Japanese Patent Application Laid-Open No. 2010-139766, a material with significant chromatic dispersion is used for a negative lens arranged in the front lens group at a position in which the height of incidence of a peripheral ray is high to correct lateral chromatic aberration in the light having a short wavelength that has been overcorrected. In Japanese Patent Application Laid-Open No. 2010-139766, a meniscus-shaped negative lens is used for the first lens on an enlargement side (enlargement conjugate side), and anomalous high-dispersion glass is used as a material of the first lens. With such a configuration, since a ray having a high image height is bent in an under direction, an effect of correcting lateral chromatic aberration can be obtained. At this time, in a case where refractive power of the meniscus-shaped negative lens is strengthened to widen an angle of view, axial chromatic aberration is generated in an over direction, and second-order dispersion in axial chromatic aberration of blue short-wavelength light tends to occur.